Experimental automatic language switching support.

With this CL, LatinIME switches the current subtype from its enabled
subtypes based on the first locale in EditorInfo#hintLocales.

This functionality is still experimental, and will be triggered only
when EditorInfo#hintLocales is specified by the application.

Bug: 22859862
Change-Id: Ibd0559b370d8aa0d50d1bada8ecfdac0ed8db898